Top-hinged ailerons

[Seraph]

I am building a model which has a fairly thick wing section with ailerons hinged along their top edges using the covering film.

My question is does this have a similar effect to using differential aileron movement. Or, to put it differently, will the up going aileron produce more additional drag than the down going one, although they have the same movement?

[simon barr]

No, it will all be as normal, but with the hinge line sealed (with the covering), they will be more effective because there will be no gap for the air to spill through. If you need differential, just set it up as normal either mechanically or with the Tx.

[Simon Chaddock]

Seraph

To be exact a top hinge is not quite the same aerodynamically as a centre line one.

With a top hinge the leading edge of the aileron (and possibly the trailing edge of the wing) has to be chamfered to provide the clearance for the downward travel.

The resulting gap reduces with down travel, thus improving the streamline, whereas it gets bigger going up and creates more drag.

So you are correct it does create a differential effect but it is pretty small and by itself will only reduce, not eliminate, adverse aileron yaw.

[Seraph]

Thank you Simon for your explanation. This is one of those thing which I felt intuitively to be so but couldn't find any theoretical support for. My own rationalisation was that if you could consider the aileron as essentially a thin flat sheet, hinged to the top surface of the wing then it was obviously going to penetrate more into the airflow above the wing than below creating more drag there. But, as one of my old maths lecturers used to say, "Obvious is what I know how to prove" and I don't. That's why I've raised the question here.

[Biggles' Elder Brother - Moderator]

One way to think about this is:

The pivot point is at the upper surface of the aileron. There is, as Simon says, a chamfer extending pretty well from the upper surface to the lower surface. As shown below:

Now suppose the up and down throw are set to the same angle and are set such that the point B comes into contact with the wing trailing edge. Under these conditions you will effectively have more up than down aileron - i.e. you will have differential. This is because the length AC is longer than the length BC, and so the area of aileron projecting into the flow when up will be bigger than the area porjected when down.

The effect, as has been said, is unlikely to be major. Typical settings for differential ailerons are often at 50% plus differences. How much difference you will get here depends of course on the angle of chamfer - but in practice its unlikely to be more than 10-15%. However it would be a greater effect for thicker section wings given the same angle.

BEB

[Seraph]

BEB, that's a very simple and elegant geometrical explanation - thank you.

[Simon Chaddock]

Actually I was thinking more about the effect on the aerodynamics than the geometry.

With the aileron up there is a big gap in the underside which seriously effects the airflow on the low pressure side of the aileron creating extra drag.

With down aileron both the upper and lower surfaces are continuous reducing the turbulence and drag although there is a small ridge.

This ridge can of course be avoided if the chamfer angle is shared equally between the wing and aileron.

But as BEB points out the overall effect is small compared to the differential required to fully counter adverse aileron yaw..

 

Edited By Simon Chaddock on 10/04/2013 00:24:37

[Seraph]

I think you, Simon and BEB have provided two distinct and consistent explanations of this.

i would now like to ask why, with conventional centre-hinged ailerons (I'm excluding balanced ailerons and Frise ailerons for the moment) you necessarily get adverse yaw.

I have been assured that this is simply the consequence of drag being proportional to lift. The lowered aileron results in more lift and, hence, more drag while the raised aileron reduces lift and drag.

Is this a correct and complete explanation?

[The Wright Stuff]

Hi Seraph,

More or less right. To make it simple. think about a symmetrical wing lifting section. If the centreline of the section was exactly aligned (inclined) to the direction of travel, then the upward going aileron would (angle for angle) produce exactly the same drag as the downward one. Hence no adverse yaw. Of course, such a wing would produce no lift, so it has to be inclined to the airflow, which of course produces lift (and drag) on both sides. Under this condition, the down going aileron acts to add to the existing drag, whereas the upward going aileron acts to negate some of the drag: hence adverse yaw is inevitable for any wing that produces a net lifting force.

This is simplified - in fact there are all sorts of viscosity and laminar flow factors too, but I believe that explanation is sufficient.

Edited By The Wright Stuff on 10/04/2013 16:58:02

Edited By The Wright Stuff on 10/04/2013 16:58:21

[will -0]

Seraph,the sources of adverse yaw are quite well covered here. You'll see that the main component of adverse yaw comes from the rolling motion of the aircraft through the air. In fact you get adverse yaw even if you don't use ailerons to roll (eg rolling a harrier using just the puffer reaction controls).

 

Edited By will -0 on 10/04/2013 17:15:38

[Seraph]

Thank you Will-o. That is very clear.